JP2002-357127A published by the Japan Patent Office in 2002 discloses an electrical supercharging device for supercharging intake air of an internal combustion engine. The device comprises a supercharger disposed in the intake passage of the internal combustion engine and an electric motor driving the supercharger. The supercharger comprises a Root's blower or a displacement compressor.
When the engine is operating at a high load, the supercharger supercharges intake air of the internal combustion engine in response to the operation of the electric motor. When the engine is operating at a low load, the device is adapted to allow natural aspiration of intake air by the engine through the supercharger. Under these conditions, the supercharger is rotated by the flow of intake air. The supercharger thereby performs power generation operations by driving the electric motor as a generator. Generated power is stored in a battery and is used in order to drive the supercharger as well as other uses. In this manner, a portion of the electrical energy used for supercharging is recovered when the engine operates at a low load.